Cold rolled and galvanized or galvannealed dual phase high strength steel and method of its production

ABSTRACT

A method of producing cold rolled and annealed dual phase high strength steel sheets, and sheets produced by the method, including hot dip galvanized and galvannealed steel sheets having a tensile strength of at least about 750 MPa and a superior balance of strength and ductility using conventional thermomechanical processing parameters robust to the processing conditions. The synergistic effect on hardenability of Cr and V, within a controlled range, facilitates production using robust processing conditions, enabling production of a high strength product having a very low yield ratio.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method of producing cold rolledannealed and hot dip galvanized and galvannealed dual phase highstrength steel sheets having tensile strength of at least about 750 MPafor use in automotive structural and other sheet steel applications, andto steel sheets produced by the method.

[0003] 2. Description of the Prior Art

[0004] The conflicting demands for lighter weight automobiles withgreater crashworthiness has led the steel industry to develop new steelspossessing very high strength combined with good ductility. To achievethis, bake hardenable and high strength IF steels have been the mostwidely used high strength thin sheets. However, their yield and tensilestrengths are generally limited to about 320 and 450 MPa, respectively.Known microalloyed steels offer even higher strength by utilizing aprecipitation hardening mechanism but their ductility significantlydecreases with higher strength. In addition, substantial amounts ofmicroalloying elements must be added to obtain tensile strength levelsin excess of 750 MPa, thus making these steels less cost-effective. Dualphase steels, by virtue of their microstructure primarily containingferrite and martensite, exhibit tensile strengths ranging from 500 to1000 MPa along with superior ductility as compared with non-dual phasesteels.

[0005] In order to realize the advantages of the high strength of dualphase steel in automotive applications, good ductility is necessary. Theyield ratio (YR) of steel, i.e., the ratio of its yield strength to itstensile strength, should be low. Typical non-dual phase steels have a YRaround 75% ± about 2% while dual phase steels typically have a YR ofapproximately 60%. A lower YR, i.e. lower yield strength and/or highertensile strengths, are desired to provide the desired formabilitydemanded by the automotive industry.

[0006] U.S. Pat. Nos. 4,615,749 and U.S. Pat. No. 4,708,748 discloses amethod of making a cold rolled dual phase structural steel sheetasserted to have excellent deep drawability, high ductility, high bakehardenability, and a non-aging property at room temperature. The steelcomprises 0.001 to 0.008 wt % of C, not more than 1.0 wt % of Si, 0.05to 1.8 wt % of Mn, not more than 0.15 wt % of P, 0.01 to 0.1 wt % of Al,0.002 to 0.05 wt % of Nb, and 0.0005 to 0.050 wt % of B provided thatthe value of Nb+10% B is in the range of 0.01 to 0.08 wt %, and ifnecessary, 0.05 to 1.0 wt % of Cr. This steel sheet is hot rolled, coldrolled and continuously annealed between Ac1 and Ac3 temperatures andcooled at an average cooling rate between 0.5 to 20 C./s in atemperature range of from the soaking temperature to 750 C. andsubsequently at an average cooling rate of not less than 20 C./s in atemperature range of from 750 C. to not more than 300 C. This patentrequires specific processing constraints and the targeted tensilestrength levels are lower than 500 MPa.

[0007] U.S. Pat. No. 5,180,449 provides a method of manufacturing agalvanized high strength steel sheet with terisile strength of not lessthan 780 MPa and a yield ratio of not more than 60%. The methodcomprises preparing a steel slab with a steel composition in the rangeof 0.08 to 0.2 wt % of C., 1.5 to 3.5 wt % of Mn, 0.01 to 0.1 wt % ofAl, 0.01 wt % or less of P, 0.001 wt % or less of S, one or both of 0.01to 0.1 wt % of Ti and 0.01 to 0.1 wt % of Nb, one or both of 0.1 to 0.5wt % Cr and 0.0005 to 0.003 wt % of B, and further hot rolling, coldrolling, recrystallization annealing, and galvanizing the steel stripwith cooling rates of not less than 5 C./s after recrystallizationannealing and between 2 to 50 C./s after galvanizing.

[0008] U.S. Pat. No. 5,356,494 discloses a method of producing a dualphase high strength cold rolled steel sheet alleged to have excellentnon aging properties at room temperature. The steel sheet comprises0.001 to 0.025 wt % of C, 0.05 to 1.0 wt % of Si, 0.1 to 2.0 wt % of Mn,0.001 to 0.2 wt % of Nb, 0.003 to 0.01 wt % of B, 0.005 to 0.1 wt % ofAl, not more than 0.1 wt % of P, not more than 0.007 wt % of N, at leastone selected from a group consisting of 0.05 to 3.0 wt % of Ni, 0.01 to2.0 wt % of Mo, and 0.05 to 5.0 wt % of Cu. This steel sheet is hotrolled, cold rolled at a reduction not lower than 60%, and annealedbetween Ac1 and Ac3 temperatures and cooled at a cooling rate between 5and 100C./s.

[0009] U.S. Pat. No. 5,123,969 discloses a method of manufacturing abake hardenable cold rolled steel sheet having dual phase structure andcomprising of 0.02 to 0.06 wt % of C, 0.6 to 1.04 wt % of Mn, 0.5% orless of Si, 0.1% or less of P, 0.1% or less of Al, 0.01% or less of N,0.1% or less of Ti, and 50 ppm or less of B. The steel sheet is hotrolled, coiled at temperatures between 560C. to 720C., cold rolled andannealed at temperatures ranging from 780C. to 900C. for less than fiveminutes, cooled in air to a temperature ranging from 650C. to 750C.followed by cooling to a temperature ranging from 200C. to 400C. at arate of 50C./s to 400C./s.

[0010] Factors responsible for achieving a good balance of high strengthand ductility in dual phase steels, available from published literature,include:

[0011] (1) The tensile strength of the dual phase steels is linearlydependent upon the amount of martensite and does not depend on thecarbon content of martensite.

[0012] (2) The dislocation substructure of dual phase steel depends uponthe proportion of martensite. With higher martensite contents, themartensite is increasingly of lath variety and greater is thedislocation density of ferrite. The increase in the dislocation densityof martensite content arises from the increasing amount of strain thathas to be accommodated when the austenite transforms to martensite.

[0013] (3) The increase in the strength of the dual phase steel withhigher martensite contents is explained as follows: the inherentstrength of the ferrite is assumed to be constant; however, the higherthe martensite content, the more cold worked, i.e., stronger due to thehigher dislocation density, is the ferrite.

[0014] (4) Increase in Mn and Cr increases the amount of martensite andthus increases the tensile strength. The yield strength first decreasesby virtue of the increase in the density of mobile dislocations, andthen increases due to the strengthening caused by the increase in volumefraction of the second phase. Silicon improves the strength andductility balance, which is believed to be due to its effect on theactivity of carbon and the density of dislocations at the ferritemartensite interface.

[0015] (5) Increasing the Cr and Mn content increases the hardenabilityof the austenite pools during intercritical annealing. Increasing Ccontent does not necessarily increase the average hardenability of theaustenite pools, since some austenite may form in regions that are notenriched with Cr and Mn. Subsequent slow cooling of these regions canproduce pearlite.

[0016] (6) The best combination of strength and ductility can beachieved by providing a completely interstitially free matrix devoid offine precipitates and a 100% conversion of austenite to martensite.

[0017] (7) Grain size plays an important role with respect to achievingthe best combination of strength and ductility. Elements such as Nb andTi form carbides and carbonitrides and prevent grain growth duringintercritical annealing.

SUMMARY OF THE INVENTION

[0018] The present invention relates to a method of producing uncoatedcold rolled and annealed steel sheet as well as coated, hot dipgalvanized or galvannealed, dual phase high strength steel sheet withtensile strength at least about 750 MPa and suitable for use inautomotive structural and other sheet steel applications. A study of theprior art available on cold rolled and galvanized and galvannealed dualphase high strength steels highlights important differences between theprior art and the present invention.

[0019] It is an object of the present invention is to provide coldrolled and annealed steel sheets and hot dip galvanized and galvannealeddual phase high strength steel sheets having a tensile strength of atleast about 750 MPa.

[0020] It is another object of the present invention to provide coldrolled and annealed dual phase high strength steel sheets, including hotdip galvanized and galvannealed steel sheets, having a tensile strengthof at least about 750 MPa with superior balance of strength andductility and that can be producing using conventional thermomechanicalprocessing parameters and that is robust to the processing conditions.

[0021] It is another object to provide such steel sheets and a method ofproducing such steel sheets having a low YR and therefore excellentformability.

[0022] Another object is to provide such a method and product in whichthe yield strength can be increased for application where a low YR isnot required without requiring a change in steel chemistry.

[0023] The object of the present invention is achieved in a cold rolledand annealed steel sheet, and hot -dip galvanized and galvannealed steelsheets having a tensile strength of at least about 750 MPa, comprising

[0024] a steel sheet containing 0.07 to 0.2% of C, 0.1 to 0.5% of Si,1.75 to 3.5% of Mn, 0.05% or less P, 0.01% or less S, 0.15 to 1.0% Cr,0.02 to 0.10% V,

[0025] 0.1% or less of Nb, 0.1% or less of Ti, 0.1% or less Al, 0.01% orless of N, and the balance Fe and incidental impurities;

[0026] a steel sheet having a microstructure primarily consisting of aferrite matrix and martensite; and

[0027] a hot dip galvanized or galvannealed layer formed on the steelsheet.

[0028] Further, the present invention provides a method for producing acold rolled and annealed steel sheet and hot dip galvanized orgalvannealed steel sheet having a tensile strength of at least about 750MPa, comprising the steps of:

[0029] rough rolling a steel containing 0.07 to 0.2% of C, 0.1 to 0.5%or less of Si, 1.75 to 3.5% of Mn, 0.05% or less P, 0.01% or less S,0.15 to 1.0% Cr, 0.02 to 0.10% V, 0.1% or less of Nb, 0.1% or less ofTi, 0.1% or less Al, 0.01% or less of N, and the balance Fe andincidental impurities;

[0030] finish rolling the rough rolled steel at a temperature not lowerthan the Ar3 point;

[0031] coiling the finish rolled steel at a temperature lower than Ar1point;

[0032] pickling the coiled hot rolled band;

[0033] cold rolling the coiled hot rolled band to the desired thicknessproducing a total reduction not less than 40%;

[0034] performing the continuous hot dip galvanizing or galvannealingcomprising the steps of:

[0035] soaking the cold rolled strip at a temperature between Ac1+20C.and Ac3;

[0036] cooling the strip to a zinc bath temperature around 470C. at aconventional cooling rate depending upon the line speed;

[0037] hot dip galvanizing or galvannealing the strip at a galvannealingtemperature around 520C.; and

[0038] cooling the galvanized or galvannealed strip to room temperatureat a conventional cooling rate.

[0039] The strip can be further subjected to temper rolling or tensionleveling to increase yield strength above that which results from theannealing treatment, up to a level approaching that of the tensilestrength, if desired for applications not requiring a low YR.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Other features and advantages of the invention will becomeapparent from the description contained herein below, taken inconjunction with the single drawing FIGURE which is a graph showing theeffect on the YR of varying the Cr and V content of the steel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] The improved cold rolled dual phase steel of the invention is arobust product produced using the conventional thermomechanicalprocessing parameters, and hence, exhibiting a uniformity in propertiesalong the length of the coil, and being more cost effective. Thechemical composition (in wt %) and the thermomechanical processingparameters are described below. The term “robust” is intended to imply aprocess in which the parameters such as annealing or rollingtemperatures do not require precise control in order to achieve thedesired high strength and low YR so that conventional processing andequipment can be employed without adversely affecting the productproperties.

[0042] C is an austenite-forming element. Higher levels of C help formmore austenite during intercritical annealing. The higher the amount ofaustenite, the higher is the amount of martensite upon cooling the stripat room temperature, assuming other factors are kept constant. Thetensile strength of the dual phase material is linearly dependent on theamount of martensite. The higher the amount of martensite, the higherthe tensile strength. Thus, increasing C increases tensile strength ofthe material. When C is below 0.07%, sufficient martensite is notobtained to achieve the minimum tensile strength of at least about 750MPa. However, increasing C above 0.20% decreases ductility and alsodeteriorates spot weldability. Hence, C is limited between 0.07% to0.20% and preferably between 0.08 and 0.16% to maintain thestrength-ductility-weldability balance.

[0043] Si is a ferrite-forming element that, in amounts above about0.1%, significantly improves the tensile strength and ductility balance.However, increasing Si above a certain limit adversely affects thecoatability of the material and is hence, limited to 0.5% or less. Inaccordance with the present invention, the amount of Si is thereforeheld within the range from about 0.1 to about 0.5%, and preferably fromabout 0.1 to about 0.35%.

[0044] Mn is an austenite-forming element. It promotes formation ofaustenite, which transforms to martensite upon cooling. Higher amount ofMn increases the amount of martensite and hence the tensile strength ofthe material. A minimum Mn content of 1.75% is required to obtain theminimum tensile strength of at least about 750 MPa. However, Mn above acertain limit, in this case, about 3.5% deteriorates the ductility aswell as the strength and ductility balance of the material. Preferably,the Mn content is maintained within the range of about 2.0 to 2.75%.

[0045] P is an impurity element and deteriorates ductility of thematerial and in amounts above 0.05% decreases the spot weldability ofthe material. A P content of less than 0.05%, and preferably within therange of 0.005 to 0.010, is employed.

[0046] S is an impurity element that combines with Mn to form MnSinclusions and reduces the ductility of the material. It alsodeteriorates the spot weldability and stretch flangeability of thematerial. The S content is therefore maintained at 0.01% or less, andpreferably within the range of 0.002 to 0.008%.

[0047] Cr is a ferrite-forming element as well as a hardenability agent.It delays the transformation of austenite to pearlite or bainite duringthe cooling of the strip after intercritical annealing and thus allowsthe austenite to transform to martensite at room temperature. Cr, thus,increases the tensile strength of the material. A minimum amount ofabout 0.15% Cr is necessary to ensure sufficient hardenability to obtainthe minimum tensile strength of at least about 750 MPa; however, a Crcontent above about 1.0% deteriorates the strength and ductility balanceof the material. Preferably the Cr content is within the range of about0.20% to about 0.55%.

[0048] V is also a hardenability agent and helps in transformation ofaustenite to martensite during cooling after intercritical annealing.The synergistic effect on hardenability produced by a combination of Vand Cr helps to produce the product using robust processing conditions.A minimum amount of V of about 0.02% is necessary to obtain sufficienthardenability to achieve the minimum tensile strength, while the higheramount of V above about 0.10% decreases the ductility of the material.Preferably the V content is within the range of about 0.03% to about0.08% to produce the desired synergistic effect with the Cr content,above.

[0049] Nb forms Nb(CN) and NbC precipitates which prevent grain growthduring hot rolling and/or intercritical annealing. Finer grain sizesincrease the strength of the material. Nb as a solute also acts as ahardenability agent and prevents transformation of austenite to otherlow temperature products other than martensite. Thus, it is veryeffective in increasing the strength and may be added when highertensile strengths around 1000 MPa are desired. However, an additionabove about 0.1% decreases the ductility of the material, and the Nbcontent should therefore not be greater than about 0.1%, and ispreferably within the range of about 0.02 to about 0.05%.

[0050] Ti combines with N and forms TiN at higher temperatures and thusprevents grain growth at higher temperatures. Finer grain size helps toincrease the strength of the material. Like, Nb, Ti may be added whenhigher tensile strengths are desired; however above about 0.1%, itadversely affects both the ductility and the surface quality of thematerial. The upper limit of Ti is therefore about 0.1%, and thepreferred Ti content is within the range of about 0.02 to 0.05%.

[0051] Al is primarily added as a deoxidizer. It also combines with N toform AIN. A high content of N decreases ductility and also causes strainaging behavior. However, if the amount of sol. Al exceeds about 0.1% theductility is decreased due to higher fraction of AIN. The upper limit ofAl is therefore about 0.1%, and the preferred Al content is within therange of about 0.02 to about 0.07%.

[0052] N should be maintained as low as practically possible, andlimited to no more than about 0.01% and preferably less than about0.005% because it decreases the ductility of the material.

[0053] Other impurities should be restricted to as small a level aspossible.

[0054] Steel with a composition described above is continuously castinto a slab of desired thickness. The slab is heated to a reheatingtemperature around 2025° F. (1107° C.). After soaking for about 45minutes, it is rough rolled and then finish rolled at a temperatureabove Ar3 to a hot rolled band. The hot rolled band is later cooled tothe desired coiling temperature, which is below the Ar1, coiled andcooled to room temperature. Higher coiling temperatures, near Ar1, arenot preferred because of the higher scale formation and the problemscaused subsequently during pickling.

[0055] The as-coiled hot rolled band is cold rolled to the desiredthickness. A minimum total reduction of about 40% is necessary to ensuresufficient stored energy of cold work in the material to produce arequired grain size after recrystallization during intercriticalannealing of the material. Higher cold reductions around 60% to 70% arepreferred to obtain a fine ferrite grain size to achieve the bestcombination of strength and ductility.

[0056] The cold rolled strip is annealed in the intercritical regionbetween Ac1+20C. and Ac3 and soaked for a time depending upon the linespeed of the strip to ensure the formation of austenite time range. Theline speed is a function of the final gage of the material. Higher linespeeds are achieved for lighter gage material. If the strip is annealedbelow the Ac1+20C. temperature, sufficient austenite is not formed atthe intercritical temperature for the given line speed and the resultingmicrostructure does not yield the desired properties due to insufficientmartensite. On the other hand, if the annealing temperature increasesabove Ac3, the ductility deteriorates sharply. The strip is later cooledfrom the intercritical annealing temperature at a conventional coolingrate of approximately 10C./s depending upon the line speed to the zincbath temperature of approximately 470C. and galvanized or galvannealed.In the case of galvannealing, the strip passes through a furnace and isheated to a temperature around 520C. The final properties areindependent of the cooling rate by virtue of sufficient hardenability inthe material. Thus, the required properties are also achieved at coolingrates below 5C./s. The galvanized or galvannealed strip is then cooledto room temperature. An uncoated cold rolled and annealed strip havingthe desired strength and ductility properties may also be produced bysimply cooling the strip from the annealing temperature to roomtemperature.

[0057] The following example further illustrates the present invention:

[0058] Steel slabs with chemical compositions given in Table 1 weremelted in the form of laboratory ingots. The balance elements not givenin Table 1 were Fe and unavoidable impurities. The slabs were hot rolledto a hot band gauge of 2.7 mm at an average finishing temperature of1650F. (899C.), and coiled at an average coiling temperature of 1050F.(566C.). The hot band was cold rolled to 1.0 mm cold rolled gaugeemploying a total reduction of 63%. The cold rolled material was latergleeble-annealed simulating a continuous hot-dip galvanizing line at anaverage soaking temperature of 1544F. (840C.). The strip was latercooled to the zinc bath temperature at a conventional cooling rate ofapproximately 7C./s to 12C./s depending upon the line speed of the stripand later galvanized. The strip was later cooled to room temperature ata cooling rate of approximately 5C./s. Table 2 indicates the processingconditions of the material. Finally, specimens were tested according tothe ASTM-L standard testing procedures. The mechanical properties fromthese specimens are indicated in Table 3.

[0059] Steels A to D having chemical composition in the range of thepresent invention and steels E and F having chemical composition out ofthe present invention range were prepared and are shown in Table 1. Thesteels were hot rolled, pickled, cold rolled, and annealed at conditionsshown in Table 2. The processed steels properties are shown in Table 3.TABLE 1 Steel C Mn Si P S Al N Nb Ti V Cr Class A 0.1 2.0 0.3 0.01 0.010.03 0.003 0.0 0.0 0.1 0.5 P B 0.15 2.25 0.02 0.01 0.01 0.02 0.003 0.0250.03 0.05 0.2 P C 0.14 2.30 0.02 0.01 0.01 0.03 0.003 0.03 0.03 0.050.45 P D 0.13 2.30 0.02 0.01 0.01 0.03 0.003 0.025 0.03 0.05 0.7 P E0.095 1.75 0.31 0.01 0.01 0.03 0.003 0.0 0.0 0.1 0.88 P F 0.095 1.65 0.30.01 0.01 0.03 0.003 0.0 0.0 0.1 0.5 C

[0060] TABLE 2 HB CR Steel (mm) FT (C) CT (C) CR (%) (mm) AT (C)(ft/min) A-D 2.7 899 566 63 1.0 840 330

[0061] TABLE 3 YS Steel (MPa) TS (MPa) YR (%) EL (%) n (4-6%) n (10-15%)A 403 820 49 17.0 0.179 0.121 B 482 986 49 15.0 0.118 — C 521 1030 5113.0 0.102 — D 548 1036 53 14.0 0.096 — E 377 791 48 18.0 0.18 0.121 F342 745 46 18.0 0.21 0.147

[0062] The effect of temper extension on the mechanical properties ofthis material was also determined with an objective to produce materialwith different yield strength for a given minimum tensile strength ofapproximately 1000 MPa. Steel C that was annealed as explained above wasfurther temper rolled at different extensions ranging from 0% to 3%.Table 4 shows the mechanical properties of steel C after differentlevels of temper extension. TABLE 4 Temper Steel EL (%) YS (MPa) TS(MPa) YR (%) EL (%) n (4-6%) C 0 521 1030 51 13.0 0.102 0.5 654 1036 6312.0 0.092 1.0 776 1069 73 12.0 0.085 1.5 869 1083 80 10.0 0.079 3.0 9251087 85 8.0 0.051

[0063] The drawing figure shows a plot of the YR for differentcombinations of Cr and V, with the Cr content, in wt %, on the x-axisand the V content on the y-axis. On this plot, the numbers in % on thebody of the graph are the yield ratio YR. As pointed out previously, alower YR, which implies a lower yield strength and higher tensilestrength, is desired because the material can stretch uniformly tohigher strains and thus provide better ductility. As is seen in thedrawing, in accordance with this invention, there is a defined region inthe V-Cr plot where the YR obtained is about 50%. The YR for differentcombinations of V and Cr are shown and the synergistic effect of the Crand V within the range described above on the YR is apparent.

[0064] The plot is divided into four regions, the first being designatedthe low yield ratio region which comprises a Cr content between 0.15%and 1.0% and a V content between 0.02% and 0.10%. At V contents below0.02%, the addition of Cr results in a YR of 60% or greater; however, atV contents above about 0.02%, the YR drops to around 50% with theaddition of Cr in amounts above about 0.15%. This decrease in YR is asignificant change, enabling the production of superiorstrength-ductility balance in these sheets.

[0065] Region 2 is a high yield ratio region comprising Cr content belowabout 0.15% and V content below 0.02%. Significantly lower yield ratios,i.e. around 50% are not obtained in this region.

[0066] Region 3 comprises a V content of above 0.1% and is notconsidered economically viable because of the high cost of V as an alloyelement.

[0067] Region 4 comprises a Cr content above 1.0% which results in aproduct having poor coatability characteristics.

[0068] While preferred embodiments of the invention have been disclosedand described, it is to be understood that the invention is not solimited, but rather that it is intended to include all embodiments whichwould be apparent to one skilled in the art and which come within thespirit and scope of the invention.

We claim:
 1. A cold rolled and annealed dual phase galvanized orgalvannealed steel sheet having a tensile strength of at least about 750MPa and a superior balance of strength and ductility, the steel sheetessentially comprising, by weight: C=0.07 to 0.2% Si=0.1 to 0.5% Mn=1.75to 3.5% Cr=0.15 to 1.0% V=0.02 to 0.1 0% P=no more than about 0.05% S=nomore than about 0.01% Nb=no more than about 0.1% Ti=no more than about0.1% Al=no more than about 0.01% N=no more than about 0.01% Balance Feand unavoidable impurities
 2. The steel sheet defined in claim 1,wherein the YR is within the range of about 50 to 54%.
 3. The steelsheet defined in claim 1, wherein the Cr content is within the range ofabout 0.20 to about 0.55%.
 4. The steel sheet defined in claim 1,wherein the V content is within the range of about 0.03 to about 0.08%.5. The steel sheet defined in claim 1, wherein the Cr content is withinthe range of about 0.20 to about 0.55% and the V content is within therange of about 0.03 to about 0.08%.
 6. The steel sheet defined in claim5, wherein the YR is within the range of about 50 to 54%.
 7. The steelsheet as defined in claim 1 having the following composition, by weight:C=0.08 to 0.16% Si=0.1 to 0.35% Mn=2.0 to 2.75% Cr=0.20 to 0.55% V=0.03to 0.08% P=0.005 to 0.010% S=0.002 to 0.008% Nb=0.02 to 0.05% Ti=0.02 to0.05% Al=0.02 to 0.07% N=0.0010 to 0.0050% Balance Fe and unavoidableimpurities
 8. A method for producing a cold rolled and annealed highstrength dual phase steel sheet having a tensile strength of at leastabout 750 MPa and a yield ratio within the range of about 50% to about54%, comprising the steps of: rough rolling a steel containing, byweight, about 0.07% to about 0.2% of C, about 0.1% to about 0.5% of Si,about 1.75% to about 3.5% of Mn, about 0.15% to about 1.0% of Cr, about0.02% to about 0.10% V, no more than about 0.05% of P, no more thanabout 0.01% of S, no more than about 0.1% of Nb, no more than about 0.1%of Ti, no more than about 0.10% of Al, no more than about 0.01% of N,and the balance Fe and incidental impurities, finish rolling the roughrolled steel band at a temperature not lower than the Ar3 point, coilingthe finished rolled steel band at a temperature lower than the Ar1point, pickling the hot rolled steel band, and cold rolling the steel toform a strip having the desired thickness, producing a total reductionof not less than 40%.
 9. The method defined in claim 8 furthercomprising the step of hot dip galvanizing or galvannealing the coldrolled strip.
 10. The method defined in claim 9, wherein the step of hotdip galvanizing or galvannealing the cold rolled strip comprises soakingthe strip at a temperature between Ac1 and Ac3, cooling the strip to acoating bath temperature of about 470° C. at a conventional cooling ratedepending on line speed, hot dip coating the strip, and cooling thecoated strip to ambient temperature at a conventional cooling rate. 11.The method defined in claim 10, wherein the coated strip is galvannealedat a conventional galvannealing temperature of about 520° C.
 12. Themethod defined in claim 8, wherein the Cr content of the steel is withinthe range of about 0.02 to about 0.55%.
 13. The method defined in claim8, wherein the V content is within the range of about 0.03 to about0.08%.
 14. The method defined in claim 8, wherein the Cr content iswithin the range of about 0.20 to about 0.55% and the V content iswithin the range of about 0.03 to about 0.08%.
 15. The method defined inclaim 8 further comprises the step of temper rolling or tension levelingthe strip to increase the yield strength to a desired level up to thatapproaching the tensile strength.
 16. The method defined in claim 8,wherein the cold rolling reduction is within the range of about 60 to70%.
 17. The method defined in claim 8, wherein the steel sheet has thefollowing composition, by weight: C=0.08 to 0.16% Si=0.1 to 0.35% Mn=2.0to 2.75% Cr=0.20 to 0.55% V=0.03 to 0.08% P=0.005 to 0.010% S=0.002 to0.008% Nb=0.02 to 0.05% Ti=0.02 to 0.05% Al=0.02 to 0.07% N=0.0010 to0.0050% Balance Fe and unavoidable impurities